The present invention is directed to imaging compositions and methods where the imaging compositions undergo a color or shade change upon exposure to energy at low intensities. More specifically, the present invention is directed to imaging compositions and methods where the imaging compositions undergo a color or shade change upon exposure to energy at low intensities and may be peeled from workpieces on which they are coated.
There are numerous compositions and methods employed in various industries to form images on substrates to mark the substrates. Such industries include the paper industry, packaging industry, paint industry, medical industry, dental industry, electronics industry, textile industry, aeronautical, marine and automotive industries, and the visual arts, to name a few. Imaging or marking typically is used to identify an article such as the name or logo of a manufacturer, a serial number or lot number, tissue types, or may be used for alignment purposes in the manufacture of semiconductor wafers, aeronautical ships, marine vessels and terrestrial vehicles.
Marking also is employed in proofing products, photoresists, soldermasks, printing plates and other photopolymer products. For example, U.S. Pat. No. 5,744,280 discloses photoimageable compositions allegedly capable of forming monochrome and multichrome images, which have contrast image properties. The photoimageable compositions include photooxidants, photosensitizers, photodeactivation compounds and deuterated leuco compounds. The leuco compounds are aminotriarylmethine compounds or related compounds in which the methane (central) carbon atom is deuterated to the extant of at least 60% with deuterium incorporation in place of the corresponding hydrido aminotriaryl-methine. The patent alleges that the deuterated leuco compounds provide for an increased contrast imaging as opposed to corresponding hydrido leuco compounds. Upon exposure of the photoimageable compositions to actinic radiation a phototropic response is elicited.
Marking of information on labels, placing logos on textiles, or stamping information such as company name, a part or serial number or other information such as a lot number or die location on semiconductor devices may be affected by direct printing. The printing may be carried out by pad printing or screen printing. Pad printing has an advantage in printing on a curved surface because of the elasticity of the pad but is disadvantageous in making a fine pattern with precision. Screen printing also meets with difficulty in obtaining a fine pattern with precision due to the limited mesh size of the screen. Besides the poor precision, since printing involves making a plate for every desired pattern or requires time for setting printing conditions, these methods are by no means suitable for uses demanding real time processing.
Hence, marking by printing has recently been replaced by ink jet marking. Although ink jet marking satisfies the demand for speed and real time processing, which are not possessed by many conventional printing systems, the ink to be used, which is jetted from nozzles under pressure, is strictly specified. Unless the specification is strictly met, the ink sometimes causes obstruction of nozzles, resulting in an increase of reject rate.
In order to overcome the problem, laser marking has lately been attracting attention as a high-speed and efficient marking method and is already put to practical use in some industries. Many laser marking techniques involve irradiating only necessary areas of substrates with laser light to denature or remove the irradiated area or irradiating a coated substrate with laser light to remove the irradiated coating layer thereby making a contrast between the irradiated area (marked area) and the non-irradiated area (background).
Using a laser to mark an article such as a semiconductor chip is a fast and economical means of marking. There are, however, certain disadvantages associated with state-of-the art laser marking techniques that burn the surface to achieve a desired mark. For example, a mark burned in a surface by a laser may only be visible at select angles of incidence to a light source. Further, oils or other contaminants deposited on the article surface subsequent to marking may blur or even obscure the laser mark. Additionally, because the laser actually burns the surface of the work piece, for bare die marking, the associated burning may damage any underlying structures or internal circuitry or by increasing internal die temperature beyond acceptable limits. Moreover, where the manufactured part is not produced of a laser reactive material, a laser reactive coating applied to the surface of a component adds expense and may take hours to cure.
Alternatively, laser projectors may be used to project images onto surfaces. They are used to assist in the positioning of work pieces on work surfaces. Some systems have been designed to project three-dimensional images onto contoured surfaces rather than flat surfaces. The projected images are used as patterns for manufacturing products and to scan an image of the desired location of a ply on previously placed plies. Examples of such uses are in the manufacturing of leather products, roof trusses, and airplane fuselages. Laser projectors are also used for locating templates or paint masks during the painting of aircraft.
The use of scanned laser images to provide an indication of where to place or align work piece parts, for drilling holes, for forming an outline for painting a logo or picture, or aligning segments of a marine vessel for gluing requires extreme accuracy in calibrating the position of the laser projector relative to the work surface. Typically six reference points are required for sufficient accuracy to align work piece parts. Reflectors or sensors are positioned in an approximate area where the ply is to be placed. Since the points are at fixed locations relative to the work and the laser, the laser also knows where it is relative to the work. Typically, workers hand mark the place where the laser beam image contacts the work piece with a marker or masking tape to define the laser image. Such methods are tedious, and the workers' hands may block the laser image disrupting the alignment beam to the work piece. Accordingly, misalignment may occur.
Another problem associated with laser marking is the potential for ophthalmological damage to the workers. Many lasers used in marking may cause retinal damage to workers. Generally, lasers, which generate energy exceeding 5 mW present hazards to workers.
Accordingly, there is a need for improved imaging compositions and methods of marking a work piece.